Question on ADIRU AIR DATA INVALID failure

[tango4 4]

Hi everyone,

There is something I can't figure out regarding this failure.

When I trigger it, here is what happens:

AP and AT disconnect. That part seems logic of course.

But I loose Air data on both PFD (Capt and FO),while ISFD is still displaying Air Data. The way I understood the FCOM, both PFD should display some data, although they should disagree.

I have the NAV AIR DATA SYS EICAS message, and the associated checklist says (First page, second item): "Crosscheck airspeed and altitude on the PFDs and standby flight instruments for accuracy. Each display is receiving data from an independent source." This is exactly the way I understood the FCOM. 

Now, let's assume this is due to the automatic behavior of the ADIRU. When I push the AIR DATA/ATT switches to ALTN on both PFD's, it is clearly stated that each PFD becomes single channel sourced (One from ADIRU, the other from SAARU), and that I should be able to see some readings on both, although different.

 

Could someone tell me if this is a bug of if I'm missing something here ?

 

All the best !

[Driverab330 74]

That's what I understand as well. However In your case you lose both airspeed display on both PFDs, I would just go straight into Unreliable Speed memory items and action the unreliable airspeed check list after the flight path of the airplane is under control.

 

Because if you look at the very end of the unreliable checklist page 10.1 on the QRH the "addition information section", it says that you may get NAV AIR DATA SYS EICAS message in the event of encountering an unreliable airspeed situation.

 

As pilots we don't know what possibilities other than " air data from one of the pitot tubes disagree with the other" may triggers the NAV AIR DATA SYS and there's always a chance that you can lose both L and R pitot tube data while the Center pitot still works and this may trigger the NAV AIR DATA SYS Eicas.

 

Not sure if it is a bug, because this type of a "bug" may well happen on the real airplane which itself is highly computerized. Therefore I would treat it as an unreliable airspeed situation and deal with it that way.

 

Hope it helps

Cheers

[Richard McDonald Woods 168]

Driverab330,

 

May we know your name? It should be in your signature.

 

You seem to be quite knowledgeable in this area.

 

Cheers, Richard

[tango4 4]

Thanks for the help.

I do agree with you, this is clearly an unreliable airspeed situation. Nevertheless, I still find it strange that with AIR DATA/ATT switches in alternate there is no data. I expected wrong data, possibly on all 3 systems, but at least some data.

After reading the FCOM I had the FEELING (I insist on this word because I might be wrong) that the purpose of that very switch was basically to force "Raw data" on PFDs to allow the pilot a possibility to decide if one is valid or not. (Otherwise, the aircraft is deciding on itself).

Any further input appreciated !

 

All the best.

[downscc 1,327]

I think your feeling is okay. I expect most air data computer errors to be significant not subtle and comparison of data sources with all the information in front of you should provide an experienced pilot with a safe alternative.  Partial panel flight is basic instrument training.

[tango4 4]

Thanks for your input, Dan.

In that case do you think it would be appropriate to submit a ticket to PMDG for such an issue in order to get their official statement ?

I'm asking you because you are a beta tester, so you might be familiar with their procedures. I don't want to be a pain in the...well, somewhere !

Thanks again.

[vr-pilot 11]

Maybe it is because PMDG "ADIRU AIR DATA INVALID" failure activation disables all the main air probes (pitot, static, temp, AoA) for BOTH "ADIRU1" (Capt=left) AND "ADIRU2" (F/O=right), but not the SAARU ("ADIRU3").

So Capt. and F/O side ADIRU air data will fail simultainously. (Attitude, heading and GS remain)

Nevertheless it is strange that it is not possible to get the SAARU air data + ATT displayed on the PFDs (Capt. or F/O) via the "AIR DATA/ATT"-ALTN switch.

Maybe it is different when "ADIRU PARTIAL FAIL" or "ADIRU SINGLE CHANNEL" failures are activated. (There are some more ADIRU/SAARU failure combinations available...)

 

Here is an interesting wiki article about the subject:

https://en.wikipedia.org/wiki/Air_data_inertial_reference_unit

[downscc 1,327]

Thanks for your input, Dan.

In that case do you think it would be appropriate to submit a ticket to PMDG for such an issue in order to get their official statement ?

I'm asking you because you are a beta tester, so you might be familiar with their procedures. I don't want to be a pain in the...well, somewhere !

Thanks again.

Nope, I wouldn't recommend a trouble ticket to get PMDG "official statement" on how something is supposed to work. The source for that is the Boeing documentation, either FCOM other other sources. On the other hand if you find a solid reference that something is amiss then that is a good ticket topic.

[tango4 4]

Maybe it is because PMDG "ADIRU AIR DATA INVALID" failure activation disables all the main air probes (pitot, static, temp, AoA) for BOTH "ADIRU1" (Capt=left) AND "ADIRU2" (F/O=right), but not the SAARU ("ADIRU3").

So Capt. and F/O side ADIRU air data will fail simultainously. (Attitude, heading and GS remain)

Nevertheless it is strange that it is not possible to get the SAARU air data + ATT displayed on the PFDs (Capt. or F/O) via the "AIR DATA/ATT"-ALTN switch.

Maybe it is different when "ADIRU PARTIAL FAIL" or "ADIRU SINGLE CHANNEL" failures are activated. (There are some more ADIRU/SAARU failure combinations available...)

 

Here is an interesting wiki article about the subject:

https://en.wikipedia.org/wiki/Air_data_inertial_reference_unit

Hi Claus, and thanks for your help.

Unfortunately, that is not how the B777 is designed.There is no ADIRU1 and 2. The architecture you are referring to is basically that of older airplanes like the B744 (3 Air data systems, each connected to an Air data computer).

On the B777, ADIRU feeds both PFDs when everything is OK, and will isolate a source if it is deemed wrong. If ADIRU fails, then SAARU will take over.

Stanby instruments are only fed by SAARU on non ISFD equipped aircraft.

 

But you are right, you see different results with the other failures.

Nope, I wouldn't recommend a trouble ticket to get PMDG "official statement" on how something is supposed to work. The source for that is the Boeing documentation, either FCOM other other sources. On the other hand if you find a solid reference that something is amiss then that is a good ticket topic.

Hi again Dan, and thanks for your time.

First I asked a B777 pilot on another forum (PSX). He told that he never had this failure in real life or in simulator training but that he basically expected to see some data. This is not hard proof but made me want to ask here.

The only data I have is the Boeing FCOM. It says:

"Air Data / Attitude (AIR DATA/ATT) Source switch

 

Off-normal position (ALTN not visible, switch out):

*the ADIRU provides air data and attitude information to the PFD and ND

*Alternate sources or the SAARU are automatically selected to replace ADIRU air data or attitude, as required.

 

ALTN-non normal position (ALTN visible, switch in). The PFD and ND alternate air data/attitude source is selected as follows:

*air data (captain): SAARU single channel

*air data (first officer): ADIRU single channel

*attitude (captain and first officer): SAARU"

 

It does not explain what this failure should do in case you leave those switches in normal position (the automatic reconfiguration algorithm is not explained) BUT it does state that if I push the switch on the captain's side, Air data comes from SAARU single channel. So I don't see why a failure called "ADIRU air data invalid" would prevent air data from being displayed, at least on the captain's side.

Do you think with those references I can send them a ticket ?

[kevinh 409]

I would ask the question anyway. What harm can it do?

[777simmer 117]

I agree...at least aks PMDG what kind of component failures are assumed with the ADIRU AIR DATA INVALID failure activated.

 

I would like to look into it, but dont have the time for that at the moment :-(

[tango4 4]

Sorry for the delay, I was a bit busy.

I just submitted a support ticket.

 

If I have an official answer I'll post it here.